1. Field of the Invention
The present invention relates to an exposure apparatus for sequentially projecting an electronic circuit pattern formed on a reticle surface onto respective shots on a wafer surface via a projection optical system in manufacturing a semiconductor element such as an IC or LSI and, more particularly, to a semiconductor manufacturing exposure apparatus having a function of, even if the reticle absorbs exposure light to thermally expand during projection exposure, immediately detecting this, converting it into a shot magnification component, and quickly correcting the shot magnification component using a magnification correction function of a projection lens.
2. Description of the Related Art
Recently, as semiconductor integrated circuit patterns such as IC and LSI patterns continue to shrink in feature size, the projection exposure apparatus is demanded for high resolution, high overlay accuracy, and high throughput.
At present, the mass production line of each LSI manufacturer tends to use an exposure apparatus having high resolution for a critical layer and an exposure apparatus having low resolution but high throughput for an uncritical layer in order to increase COO (Cost Of Ownership). To cope with a process of the Mix and Match method using different apparatuses, variations in magnification and distortion in the shot must be suppressed in addition to shifts in the shot matrix, magnification errors, and rotation errors on the wafer.
In particular, as for variations in the shot, variations in magnification and distortion by thermal expansion of the reticle upon absorbing illumination light have recently surfaced. Since the reticle is made of silica glass, the glass itself has an absorption index of several % or less for exposure light (KrF 243 nm, i-line, g-line, and the like) and a low linear expansion coefficient of 0.5 ppm/xc2x0 C., and thus thermal expansion of a conventional reticle does not pose any problem. However, the Cr pattern absorbs a large amount of exposure energy because of high luminance of an illumination lamp for increasing throughput or a three-layered Cr surface for preventing flare of an optical system. The silica glass rises in temperature during a heat conduction process to thermally expand.
To avoid this phenomenon, the temperature rise is avoided by spraying conditioned air on the reticle. However, this is not practical because the temperatures of the air and reticle cannot be made equal, the apparatus becomes bulky, and the effects are low for a reticle formed with a pellicle.
There is proposed another method (e.g., Japanese Patent Laid-Open No. 4-192317) of estimating the thermal deformation amount of the reticle by numerical calculation such as the difference calculus or finite-element method on the basis of various exposure parameters (reticle surface illuminance, pattern density, and the like), and correcting the magnification or distortion component using the correction means of a projection optical system. However, this method is basically an open-loop correction. The thermal deformation amount of the reticle is difficult to estimate by numerical calculation for the use of various modified illuminations, phase shift reticles, pellicle-formed reticles, or the like in an actual process, and the use of a combination of them.
Accordingly, a so-called closed-loop correction method of directly measuring and correcting thermal deformation of the reticle is adopted. That is, thermal deformation of the reticle is measured using an existing position measurement mark used to align the reticle by a reticle stage, and is corrected based on the result using the distortion correction means of the projection optical system.
This method will be briefly explained with reference to FIGS. 3 to 5. In FIG. 3, reference numeral 1 denotes a reticle; and 2 and 3, position measurement marks formed on the lower surface of the reticle. In FIG. 4, reference numeral 4 denotes a base which supports a reticle stage (not shown); and 5 and 6, reference marks formed on the base at positions where they face the reticle position measurement marks 2 and 3. In measuring thermal deformation of the reticle, shifts (xcex94x, xcex94y) of the position measurement marks 2 and 3 are measured with reference to the reference marks 5 and 6, as shown in FIG. 5. From this result, the magnification component generated on the reticle is corrected using the distortion correction means of the projection optical system.
This method is, however, unsatisfactory.
A process for an actual element does not always use the maximum image field (e.g., 22 mmxc3x9722 mm) of the exposure apparatus. The image field is often limited by a masking device to a rectangular shape such as two or three chips by one shot depending on the chip size. In this case, according to the above method, since the edge of an actual shot is apart from the position measurement mark, the magnification variation in the shot is different from the magnification variation of the position measurement mark.
FIG. 6 shows this state. In FIG. 6, reference numeral 7 denotes a rectangular actual element area. FIG. 7 schematically shows the temperature distribution and deformation when exposure light is incident on this area to attain a thermally steady state. In FIG. 7, reference numeral 1a denotes a deformed reticle; and 2a and 3a, displaced disposition measurement marks. If the circuit pattern region has a rectangular shape long in the y direction, like this example, the temperature distribution has an elliptical shape long in the y direction, and thermal deformation along with this also becomes prominent in the y direction. As for the shot magnification variation, the y-direction magnification component greatly changes. Despite this, since the reticle position measurement marks 2a and 3a are positioned near the edge of the reticle, the magnification component calculated from the mark displacement does not reflect an actual shot magnification.
When a circuit pattern region 8 is much smaller than the maximum illumination region, as shown in FIG. 8, the temperature distribution has a small concentric shape, as shown in FIG. 9. For the same reason, the magnification component calculated from the mark displacement does not reflect an actual shot magnification, either.
As described above, the conventional method cannot accurately monitor the shot magnification variation using the mark magnification variation.
The present invention has been made to solve the above problems, and has as its object to provide an exposure apparatus and exposure control method capable of accurately estimating magnification variations in an exposure region regardless of changes in shape of the exposure region.
More specifically, when the shot magnification component generated by thermal deformation of a reticle is to be calculated from the displacement of a position measurement mark on an existing reticle, the shot magnification component can be accurately estimated even if the exposure region changes to an arbitrary size by a masking device.
It is another object of the present invention to correct the shot magnification component generated on the reticle from the estimation result using the magnification correction means of a projection lens.
It is still another object of the present invention to accurately, easily estimate magnification variations in an exposure region having an arbitrary shape by using either one of the aspect ratio and area of the exposure region as shape information of the exposure region.
It is still another object of the present invention to increase the exposure precision by correcting the magnification by the projection lens on the basis of the estimated magnification variation.
It is still another object of the present invention to execute magnification correction and minimize a decrease in throughput by execution of magnification correction when the estimated magnification variation reaches a level that poses problems in a required exposure precision.
It is still another object of the present invention to execute estimation of the magnification variation for every predetermined exposure amount in units of wafers or shots, thereby minimizing a decrease in throughput by execution of magnification correction.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.